The present invention relates to a safety device for an explosive charge and more particularly to a device for safe arming an explosive charge of the type having a detonating cord adjacent thereto which detonating cord can be actuated by a blasting cap positioned adjacent to the detonating cord.
Explosive charges are widely and effectively used within perforating devices for well bore completions of oil wells. Typically these explosive charges are mounted within a suitable strip which is then affixed within a tubular housing or carrier. Collectively these components are referred to as a perforating gun, which gun is then lowered into a borehole on a wireline or other conveyance apparatus. Either a single charge or multiple charges can be mounted within each strip, in a variety of configurations, and with a variety of spacings in between. A detonating cord or primacord for detonating the charges is secured in the vicinity of the rear of each charge and a detonator or blasting cap is secured adjacent one end of the primacord for detonating the primacord. Typically the blasting cap or detonator includes a primary explosive such as lead azide which is ignited when an igniter wire positioned therein is heated by an electrical current. The primary explosive, in turn, sets off a secondary or booster explosive, such as RDX, which develops sufficient energy to detonate the explosive in the primacord, also typically RDX. An electrical current is typically passed from the surface of the ground through the electrical conductors of a cable or the wireline to the igniter wire in the blasting cap. Passage of current through the igniter wire then initiates the aforementioned chain reaction which subsequently detonates the explosive charge.
It will be appreciated that the components of such perforating systems, i.e., the blasting cap, primacord, and explosive charges when interconnected are capable of causing severe damage if accidentally detonated. Accordingly considerable effort has already been expended in developing techniques for "safe-arming" the explosive charges such that the perforating apparatus is able to be safely handled without unplanned detonation.
One type of safe-arming system which has been developed is referred to as a barrier pin safe-arm system. This system positions the primacord and the detonator with a predetermined spacing there between. When the system is safe-armed this space between the primacord and the detonator is occupied by a specialized plug which isolates the primacord from the shock of the detonation of the detonator and thereby imposes a barrier to the detonation of the primacord. Although such a system is generally effective in safe arming the explosive charge, it requires that a specialized adapter be added to the tubular housing of the perforating gun, in order to accurately mount the primacord and detonator. This adapter includes a porthole through its side which is adapted to receive the specialized plug and which affords the extension of the plug through the porthole and into the space between the primacord and the detonator. The adapter must provide an adequate space for the primacord and detonator to be rigidly secured with an adequate separation therebetween to afford the prevention of the transmission of any detonator explosion to the primacord. This distance is large compared to the wall thickness of the standard carrier, and it is therefore very difficult to seal the interface between the porthole and the plug. Since the perforating system must typically operate in a high pressure and fluid environment this interface must however remain water-tight. This need for a water-tight seal is an additional reason for the specialized adaptor which incorporates a substantially thicker housing in order to achieve the seal. Since the plug is providing the barrier for the detonation, the positioning of the primacord and the detonator with respect to the porthole is very critical. This accurate positioning cannot typically be obtained within the standard gun carrier and this provides another reason for the specialized adaptor. The specialized adapter not only introduces additional cost and complexity into the perforating system, it also adds length to the gun which can cause difficulty when inserting the gun into, or when removing the gun from the borehole.